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  • 1
    Electronic Resource
    Electronic Resource
    Solar Convection (1990)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Astronomy and Astrophysics 28 (1990), S. 263-303 
    Details
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.aa.28.090190.001403
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  • 2
    Electronic Resource
    Electronic Resource
    Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480 (2001)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 414 (2001), S. 180-182 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Black holes become visible when they accrete gas, a common source of which is a close stellar companion. The standard theory for this process (invoking a ‘thin accretion disk’) does not explain some spectacular phenomena associated with these systems, such as their X-ray variability ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/35102515
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  • 3
    Electronic Resource
    Electronic Resource
    Is there a weak mixed polarity background field? Theoretical arguments (1987)
    Springer
    Solar physics 110 (1987), S. 115-128 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A number of processes associated with the formation of active regions produce ‘U-loops’: fluxtubes having two ends at the photosphere but otherwise still embedded in the convection zone. The mass trapped on the field lines of such loops makes them behave in a qualitative different way from the ‘omega-loops’ that form active regions. It is shown that U-loops will disperse though the convection zone and form a weak (down to a few gauss) field that covers a significant fraction of the solar surface. This field is tentatively identified with the inner-network fields observed at Kitt Peak and Big Bear. The process by which these fields escape through the surface is described; a remarkable property is that it can make active regions fields apparently disappear in situ. The mixed polarity moving magnetic features near sunspots are interpreted as a locally intense form of this disappearance by escape of U-loops.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148207
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  • 4
    Electronic Resource
    Electronic Resource
    Convective collapse of flux tubes (1979)
    Springer
    Solar physics 61 (1979), S. 363-378 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Flux tubes of constant β extending vertically through the solar convection zone are unstable to a convective instability if the surface field strength is less than 1270 G. By downward displacement of matter along the tube an unstable tube can transform into a new equilibrium state with lower energy which has a higher field strength. Numerical calculations of these ‘collapsed’ states are presented. If the collapse starts in a field with a strength corresponding to equipartition with kinetic energy in the convection zone, it yields a surface field strength of about 1650 G. It is proposed that the small scale magnetic field in active regions consists of such tubes. The collapsed state is not in thermal equilibrium. In the deeper layers the heat exchange following the collapse is very slow but the surface layers return rapidly to temperature equilibrium. It is argued that during the gradual thermal evolution of the collapsed state its surface layers may start an overstable oscillation. A brightness-velocity correlation in this oscillation could account for the observed downdraft in the tubes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00150420
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  • 5
    Electronic Resource
    Electronic Resource
    Pressure equilibrium and energy balance of small photospheric fluxtubes (1976)
    Springer
    Solar physics 50 (1976), S. 269-295 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Field configurations and temperature distributions of axially symmetric fluxtubes are determined on the basis of pressure equilibrium and energy balance of the tubes. The description concentrates on layers below ≈ 600 km above the photosphere; a magnetostatic field, and energy transport by a diffusion process are assumed. It is assumed also that the magnetic field of the tubes prevents convective flow across the field lines, so that only radiative energy exchange between the tube and the convection zone is present. A set of model tubes is presented ranging in size from facular points (150 km) to small pores (1000 km), for different values of the field amplitude and the asymptotic energy flux F 0 flowing along the tube from the deeper layers. Radial influx of heat into the tube at the photospheric level influences the temperature in the tube strongly for all these models. For a pore-like tube f 0 = 0.25 (similar to the flux from a spot umbra) seems appropriate (F 0 in units of the normal photospheric flux). If in the smallest fluxtubes F 0 is also 0.25, a comparison of the intensity contrast with observations of facular points indicates that the radius of tubes corresponding to facular points is 50–100 km. In the continuum the structure looks like a depression in the photosphere (similar to the Wilson depression of spots). The magnitude of this depression is ≈ 200 km for pores of 1000 km diameter and ≈ 100 km for facular points. The walls of the hole created by the depression contribute considerably to the contrast of structures observed near the solar limb. It is shown how this contribution may explain the centre to limb behaviour of facular contrast as seen in the continuum, and why the continuum CLV differs so strongly from that in line cores. Over the first 400 km above the photosphere the tube expands by a factor of ≈ 2 for all the tubes calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155292
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  • 6
    Electronic Resource
    Electronic Resource
    Heat flow near obstacles in the solar convection zone (1977)
    Springer
    Solar physics 55 (1977), S. 3-34 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Disturbances in the heat flow in the solar convection zone are calculated with a turbulent thermal diffusion coefficient based on a mixing length approximation. As a consequence of the radiative boundary condition at the surface and the strong increase of the diffusion coefficient with depth, the convection zone resembles a thermally superconducting shell enclosed between a thin surface layer and an interior core of low thermal conductivity. Thermal disturbances originating in the convection zone do not penetrate into the interior, and penetrate only weakly through the solar surface. A thermally isolating obstacle buried entirely in the convection zone casts a ‘shadow’ of reduced temperature at the solar surface; the brightening surrounding this shadow is undetectable. The shadow is weak unless the object is located close to the surface (less than 2000 km). Assuming a sunspot to be an area of reduced thermal conductivity which extends a finite depth into the convection zone, the heat flow around this obstacle is calculated. The heat flux blocked below the spot (‘missing flux’) spreads over a very extended area surrounding the spot. The brightening corresponding to this ‘missing flux’ is undetectable if the reduction of the thermal conductivity extends to a depth greater than 1000 km. It is concluded that no effect other than a decrease of the convective efficiency is needed to explain the temperature change observed at the solar surface in and around a sunspot. The energy balance is calculated between magnetic flux tubes, oriented vertically in the solar surface, (magnetic elements in active regions and the quiet network) and their surroundings. Near the visible surface radiation enters the tube laterally from the surrounding convection zone. The heating effect of this influx is important for small tubes (less than a few arcseconds). Due to this influx tubes less than about 1″ in diameter can appear as bright structures irrespective of the amount of heat conveyed along the tube itself. Through the lateral influx, small tubes such as are found in the quiet network act as little ‘leaks’ in the solar surface through which an excess heat flux escapes from the convection zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00150871
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  • 7
    Electronic Resource
    Electronic Resource
    A model of the solar convection zone (1974)
    Springer
    Solar physics 34 (1974), S. 277-290 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A model of the convection zone is presented which matches an empirical model atmosphere (HSRA) and an interior model. A mixing length formalism containing four adjustable parameters is used. Thermodynamical considerations provide limits on two of these parameters. The average temperature-pressure relation depends on two or three combinations of the four parameters. Observational information on the structure of the outermost layers of the convection zone, and the value of the solar radius limit the range of possible parameter combinations. It is shown that in spite of the remaining freedom of choice of the parameters, the mean temperature-pressure relation is fixed well by these data. The reality of a small density inversion in the HSRA model is investigated. The discrepancy between the present model and a solar model by Mullan (1971) is discussed briefly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00153665
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  • 8
    Electronic Resource
    Electronic Resource
    Propagation speeds and acoustic damping of waves in magnetic flux tubes (1982)
    Springer
    Solar physics 75 (1982), S. 3-17 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Propagation speeds are derived for the wave modes of a thin magnetic tube in an otherwise homogeneous magnetized or unmagnetized fluid. These results generalize results obtained by previous authors. There are three types of wave, a (torsional) Alfvén wave and two waves which are specific for the thin tube. These are named the longitudinal and transversal tube waves, according to their polarization properties. They can be camped by radiating an MHD or acoustic wave into the surroundings of the tube. Conditions for occurrence of this acoustic damping, and the damping rates, are derived. The behavior of the waves in the solar convection zone and corona is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00153456
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  • 9
    Electronic Resource
    Electronic Resource
    Internal rotation of the Sun (1983)
    [s.l.] : Nature Publishing Group
    Nature 304 (1983), S. 520-522 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] In principle the rotation rate O can be a function of both radius (r) and latitude (?). However, it has been conjectured3 that if O is not independent of ?, finite amplitude shear instability will rapidly redistribute angular momentum along equipotential surfaces, so as to make O constant on such ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/304520a0
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  • 10
    Electronic Resource
    Electronic Resource
    Magnetic flux tubes on the Sun (1983)
    [s.l.] : Nature Publishing Group
    Nature 304 (1983), S. 401-406 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Magnetic fields are the cause of almost all forms of solar activity. Near the solar surface, and possibly in the entire convection zone, these fields occur in the form of isolated flux tubes. In recent years, new views have been developed (and older ones revived) in which this property plays a ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/304401a0
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